1. Field of the Invention
The present invention relates in general to the field of signal processing, and, more specifically, to a system and method for modulating baseband noise and using filters to reduce noise in a baseband that occurs, in part, due to non-ideal system properties that mix noise into a baseband via fold back mechanisms.
2. Description of the Related Art
Many electronic systems employ signal processing technology to process analog, digital, or a mix of analog and digital signals. Components utilized to implement signal processing technology often generate unwanted noise. For example, digital-to-analog converters (hereinafter, “DAC”) are widely utilized to convert digital signals into analog signals. In the process of conversion, DACs often generate noise through quantization errors (“quantization noise”) and low frequency l/f noise.
In audio applications, the digital to analog conversion process often involves oversampling a digital signal, modulating the signal using a delta-sigma modulator to shape noise associated with quantizing the digital signal, and performing a digital to analog conversion using a low-pass filter. The filtered output signal is generally amplified to produce an analog signal suitable for driving a load. Delta-sigma modulators receive an input signal and convert the signal into a series of binary pulses having an average amplitude over time proportional to the input signal. In the process of producing a modulated output signal, delta-sigma modulators introduce quantization noise into the modulated input signal. However, the quantization noise advantageously resides outside of the audio baseband where frequency components of interest reside, i.e. between about 0 Hz and above about 20-25 kHz. Nevertheless, some post modulation processing, such as a post-modulation digital to analog conversion and low pass filtering, introduces noise into the audio baseband.
One common type of noise generated in post modulation processing circuits, such as metal oxide semiconductor gain stages, is l/f noise which, as the nomenclature implies, has relatively high energy at low frequencies that rapidly diminishes at higher frequencies. Analog filters often include one or more gain stages that introduce l/f noise. A modulation technique referred to as “chopping” has been implemented in conventional technology to modulate l/f noise out of the audio baseband.
FIG. 1 depicts a chopping circuitry and amplifier 100, which is utilized as a component in many well-kmown circuits such as switched capacitor digital to analog converters. The input signal x1(t) is modulated by chopper circuit 102 at a frequency fchop for a chopper control signal c(t). FIG. 2A depicts modulated input signal X1(f) in the frequency domain, centered on fchop, and harmonics of, fchop. The amplitude of the modulated input signal X1(f) decreases with l/n, where n is the harmonic number. The baseband of X1(f) extends to frequency fB, which in audio applications is about 20-25 kHz. The l/f noise is added to the modulated input signal x1(t) after chopping.
FIG. 2B illustrates the l/f noise in the frequency domain. As mentioned above, the energy of the l/f noise is primarily located within low frequencies, including the baseband of audio signals. Gain stage 104, which may be part of a larger circuit (not shown), such as a low pass filter, amplifies the modulated input signal x1(t) and l/f noise.
Chopper circuit 106 demodulates the output signal of gain stage 104 at the frequency of chopper signal c(t) to produce output signal x2(t). FIG. 2C depicts signal x2(t) and the l/f noise signal in the frequency domain.
The demodulation of signal x1(t) moves the output signal of interest, x2(t), in the frequency domain back to the baseband and centers the l/f noise at fchop and harmonics thereof, thus out of the baseband. In an audio application, a low pass filter (not shown) attenuates signals having frequency components of x2(t) outside fB.
U.S. Pat. No. 4,939,516 issued Jul. 3, 1990 and entitled “Chopper Stabilized Delta-Sigma Analog-to-Digital Converter”, Early et al inventors (hereinafter, “Early”), and U.S. Pat. No. 5,039,989 issued Aug. 13, 1991 and entitled “Delta-Sigma Analog-to-Digital Converter with Chopper Stabilization at the Sampling Frequency,” Welland et al inventors (hereinafter, “Welland”), describe conventional applications of chopping in analog-to-digital converters (hereinafter “ADC”). Early and Welland proposed solutions using chopping circuitry to address l/f and other noise issues that are particularly unique to ADCs.
Early proposed synchronizing a chopping frequency with an analog input signal sampling frequency and a digital filter. Early selected a chopping frequency equal to one-half of an analog input signal sampling frequency so that the chopping frequency would be in the rejection portion of the digital filter's frequency response. Early considered such synchronization to be important because the digital filter was able to provide a rejection of the l/f noise that was modulated to the chopping frequency. See, for example, Early, col. 8, Ins. 37-64.
Welland recognized that, in an ADC, choosing a chopping frequency equal to one-half of the sampling frequency of an analog input signal can actually increase the modulation of noise into an input signal's baseband. Thus, Welland selected a chopping frequency equal to the sampling frequency of the analog input signal. Welland included an analog modulator with at least one stage of amplification having a frequency response that provides a substantial amount of attenuation at the sampling frequency of the analog modulator in the Welland design. Thus, the amplification stage attenuates l/f noise, which is shifted up in frequency by chopping to the sampling frequency.
DACs are in many ways very different from ADCS. Consequently, chopping frequencies of DACs are selected for different reasons than ADCs. Conventional technologies implement chopping in DACs at a chopping frequency that is as low as possible relative to a digital sampling frequency. The DAC chopping frequency is conventionally chosen just high enough to shift l/f noise out of the baseband of the input signal in order to minimize parasitic effects associated with chopping circuitry.
Some DACs generate quantization noise, such as DACs that include a delta-sigma modulator and a switched capacitor DAC. The quantization noise associated with each bit can be significant enough to cause nonlinearity problems when the switched capacitor DAC receives the 1-bit and converts it into an analog signal. To address this problem, finite impulse response (FIR) filters receive the output bit of the delta-sigma modulator and produce attenuation bands, also referred to as “notches”, in the frequency domain at various divisions of an input signal sampling frequency fS. For example, notches placed at fS/n, where “n” equals 16, 8, and 4, reduce quantization noise and thus minimize or eliminate nonlinear slewing of gain circuitry in the switched capacitor DAC. Embodiments of the switched capacitor DAC processing 1-bit from the delta-sigma modulator at a time also include chopping circuitry with a frequency fchop equal to fS/n.